The fire was reported to be inside a catalytic cracker. It's described as a "a chemical reactor for converting oils with high boiling points into fuels with lower boiling points in the presence of a catalyst."

From the refinery's website:

The Delaware City refinery is located on a 5,000-acre site on the Delaware River, with the ability to accept crude by rail or waterborne cargoes. Delaware City possesses an extensive distribution network of pipelines, barges and tankers, truck and rail for the distribution of its refined products.